1. Field of the Invention
This invention relates to an output circuit for outputting an IF count signal for an FM IF amplifier.
2. Description of the Related Art
First, the background art circuit will be explained, with reference to FIGS. 3 and 4. In the circuit of FIG. 3, an IF signal supplied to input terminal 1 is amplified by means of IF amplifier 2, and then supplied to output terminal 3, which is connected to power source terminal Vcc via load resistor 4. Gate circuit 5 provided separately from IF amplifier 2 is supplied with an IF count signal. The IF count signal is subsequently derived from output terminal 7 of gate circuit 5 which is controlled by a control signal supplied to control terminal 6.
However, with the circuit construction shown in FIG. 3, the amplified IF signal always appears on output terminal 3, and tends to interfere with the IF input signal and/or signals in peripheral circuits. As a result, the operation of IF amplifier 2 is liable to become unstable and affect the operation of the peripheral circuits.
To solve the above problem, IF amplifier 2 and gate circuit 5 of the circuit of FIG. 4 are formed in a single IC 8. In this Figure, portions which are the same as those in FIG. 3 are denoted by the same reference numerals. With the circuit of FIG. 4, an IF count signal is normally interrupted, and when required, a control signal is supplied to gate control terminal 6 so as to permit the IF count signal to be derived from output terminal 7. Therefore, the IF input signal and signals in the peripheral circuits will not be interfered.
However, in the circuits of FIGS. 3 and 4, two terminals, that is, gate control terminal 6 and output terminal 7 for the IF count signal, are essentially required and, therefore, it is difficult of reduce the number of terminals necessary to the IC.
As described above, in the background art output circuit, it is necessary to use the gate control terminal and IF count signal output terminal, preventing the number of terminals from being reduced.